<!DOCTYPE html>
<html>

<head>
    <title>Bootstrap 表格</title>
    <meta charset="utf-8">
    <!-- 适应移动设备 -->
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="./css/bootstrap.css">
    <script src="./jquery-3.5.1.min.js"></script>
    <script src="./js/bootstrap.js"></script>
</head>

<body>
    <br>
    <div class="container" style="border: 1px solid red;">
        <div class="dropdown">
            <button type="button" class="btn dropdown-toggle" id="dropdownMenu1" data-toggle="dropdown">主题
                <span class="caret"></span>
            </button>
            <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">Java</a>
                </li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">数据挖掘</a>
                </li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">数据通信/网络</a>
                </li>
                <li role="presentation" class="divider"></li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">分离的链接</a>
                </li>
            </ul>
        </div>
    </div>
    <br>
    <div class="container" style="border: 1px solid red;">
        <div class="dropdown">
            <button type="button" class="btn dropdown-toggle" id="dropdownMenu1" data-toggle="dropdown">
                主题<span class="caret"></span>
            </button>
            <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">
                <li role="presentation" class="dropdown-header">下拉菜单标题</li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">Java</a>
                </li>
                <li role="presentation" class="disabled">
                    <a role="menuitem" tabindex="-1" href="#">数据挖掘</a>
                </li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">数据通信/网络</a>
                </li>
                <li role="presentation" class="divider"></li>
                <li role="presentation" class="dropdown-header">下拉菜单标题</li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">分离的链接</a>
                </li>
            </ul>
        </div>
    </div>

    <br>
    <div class="container" style="border: 1px solid red;">
        <div class="dropdown dropup">
            <button type="button" class="btn dropdown-toggle" id="dropdownMenu1" data-toggle="dropdown">
                向上弹出<span class="caret"></span>
            </button>
            <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">
                <li role="presentation" class="dropdown-header">下拉菜单标题</li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">Java</a>
                </li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">数据挖掘</a>
                </li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">数据通信/网络</a>
                </li>
                <li role="presentation" class="divider"></li>
                <li role="presentation" class="dropdown-header">下拉菜单标题</li>
                <li role="presentation">
                    <a role="menuitem" tabindex="-1" href="#">分离的链接</a>
                </li>
            </ul>
        </div>
    </div>
</body>

</html>